Structure of Bone and Cartilage Coverings

Layers and Cell Types

• Periosteum: The outer covering of bone, consisting of an outer fibrous layer (dense irregular connective tissue) and an inner cellular layer (osteogenic cells).

• Endosteum: Thin membrane lining the medullary cavity of bones, containing osteoblasts and osteoclasts.

• Perichondrium: Dense irregular connective tissue surrounding cartilage (except at joints), containing chondroblasts for growth and repair.

• Cell Types:

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells maintaining bone tissue.

  • Osteoclasts: Bone-resorbing cells.

  • Chondroblasts: Cartilage-forming cells.

  • Chondrocytes: Mature cartilage cells.

Cartilage Growth

Interstitial vs. Appositional Growth

• Interstitial Growth: Cartilage expands from within as chondrocytes divide and secrete new matrix.

• Appositional Growth: New layers of cartilage are added to the surface by chondroblasts in the perichondrium.

• Example: Interstitial growth is important during early development; appositional growth is more significant in later stages.

Bone Marrow

• Red Marrow: Site of hematopoiesis (blood cell formation), found in spongy bone of children and some adult bones (e.g., pelvis, sternum).

• Yellow Marrow: Stores fat, found in the medullary cavity of long bones in adults.

Types of Bone

Spongy (Cancellous) vs. Compact (Cortical) Bone

• Spongy Bone: Porous, contains trabeculae, found at ends of long bones and inside flat bones.

• Compact Bone: Dense, forms the outer layer of bones, provides strength and protection.

• Example: Femur has a compact bone shaft and spongy bone at the epiphyses.

Vascularization of Bone

• Bones are highly vascularized, with nutrient arteries entering through nutrient foramina to supply bone cells.

Bone Cells

Blast vs. Cyte vs. Clast

• Osteoblasts: Synthesize bone matrix.

• Osteocytes: Maintain bone tissue.

• Osteoclasts: Break down bone matrix for remodeling and calcium release.

Ossification

Endochondral vs. Intramembranous Ossification

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; most bones develop this way.

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue; forms flat bones like the skull.

• Type of Cartilage in Endochondral Ossification: Hyaline cartilage.

• Primary vs. Secondary Ossification Centers: Primary centers appear in the diaphysis (shaft); secondary centers appear in the epiphyses (ends).

Role of Periosteal Bud

• The periosteal bud brings blood vessels, nerves, and osteogenic cells into the cartilage model during endochondral ossification, initiating bone formation.

Bone Growth

Appositional vs. Interstitial

• Appositional Growth: Increase in bone diameter by addition of new bone tissue at the surface.

• Interstitial Growth: Increase in bone length at the epiphyseal plate.

Functional Zones of the Growth (Epiphyseal) Plate

• Resting (quiescent) zone

• Proliferation (growth) zone

• Hypertrophic zone

• Calcification zone

• Ossification (osteogenic) zone

Hormones Affecting Bone Growth

• Growth hormone, thyroid hormone, and sex hormones (estrogen, testosterone) regulate bone growth and epiphyseal plate closure.

Substances Used by Osteoclasts

• Osteoclasts secrete acids (e.g., HCl) and lysosomal enzymes to dissolve bone matrix and release minerals.

Wolff's Law

• Bones adapt to the loads under which they are placed; increased stress leads to bone strengthening.

Bone and Calcium Homeostasis

Role of Calcitonin & Parathyroid Hormone; Feedback Mechanism

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts and increasing calcium reabsorption in kidneys.

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity.

• Feedback Mechanism: Negative feedback regulates blood calcium levels.

• Equation:

Fracture Classification and Common Types

• Simple (closed): Bone breaks cleanly but does not penetrate skin.

• Compound (open): Bone ends penetrate the skin.

• Comminuted: Bone fragments into several pieces.

• Greenstick: Incomplete break, common in children.

• Spiral: Ragged break due to twisting forces.

Fracture Healing

• Hematoma Formation: Blood clot forms at fracture site.

• Fibrocartilaginous Callus: Soft callus forms as fibroblasts and chondroblasts invade.

• Bony Callus: Osteoblasts form new bone.

• Bone Remodeling: Bone is reshaped to original form.

Osteoporosis vs. Osteomalacia

• Osteoporosis: Decreased bone mass and density, increased fracture risk.

• Osteomalacia: Softening of bones due to defective mineralization (often from vitamin D deficiency).

Joint Structure and Movement Classifications

• Fibrous Joints: Bones joined by dense connective tissue (e.g., sutures).

• Cartilaginous Joints: Bones joined by cartilage (e.g., synchondrosis, symphysis).

• Synovial Joints: Bones separated by a fluid-filled cavity, allowing free movement.

Cartilaginous Joint Types

• Synchondrosis: Hyaline cartilage unites bones (e.g., epiphyseal plate).

• Symphysis: Fibrocartilage unites bones (e.g., pubic symphysis).

Function of Articular Cartilage

• Reduces friction and absorbs shock in synovial joints.

Synovial Joint Structure and Features

• Articular cartilage, joint (synovial) cavity, articular capsule, synovial fluid, reinforcing ligaments, nerves, and blood vessels.

Shoulder Joint Features

• Ball-and-socket joint; allows greatest range of motion but is less stable.

Sprain vs. Dislocation

• Sprain: Ligament is stretched or torn.

• Dislocation: Bones are forced out of alignment at a joint.

Composition of Connective Tissue Matrices

• Matrix consists of ground substance (proteoglycans, glycosaminoglycans) and fibers (collagen, elastic, reticular).

Common Joint Injuries

• Tendinitis: Inflammation of a tendon.

• Bursitis: Inflammation of a bursa (fluid-filled sac reducing friction).

Cause of Gout

• Deposition of uric acid crystals in joints, leading to inflammation and pain.

Summary Table: Types of Joints